The invention relates to a method and a device for tightening screw joints by the application of a number of succeeding torque impulses. In particular, the invention concerns a method which is intended for controlling and quality checking of impulse tightening processes and which is based on the determination of the installed torque in the screw joint at each one of the applied torque impulses.
A problem concerned with prior art techniques in this field is the difficulty to obtain an accurate measurement of the installed torque and, hence, an accurate final tightening level in the screw joint based on such measurement. One of the reasons behind this problem used to be the lack of reliable torque transducers suitable for torque impulse tools. Although the transducer problem nowadays has been solved, the accuracy problem as regards the installed torque measurement still exists.
Accordingly, in previously described screw joint tightening methods using torque impulse tools, as described for instance in U.S. Pat. No. 5,366,026, the torque delivered by the tightening tool is used for determining the pretension level in the screw joint. The actual torque level during the tightening process has always been determined by measuring the peak values of the delivered torque impulses, and the tightening process has been controlled by comparison of the per impulse increasing peak value with a predetermined value corresponding to a desired tension level in the screw joint.
This previously described tightening control method, however, still suffers from accuracy problems. One of the reasons is that the torque peak value indicated at each delivered impulse does not correctly reflect the true actual tension level in the screw joint. After a thorough study of the torque impulse application on screw joints, it has been established that the peak of a delivered torque impulse occurs at the beginning of the torque pulse, and that the screw joint continue to rotate over a further angular distance after that. When the screw joint actually stops rotating, the torque level is in fact substantially lower than the indicated peak value. Since the tension in the screw joint via the pitch of the thread corresponds directly to the angular displacement of the screw, the tension increases as long as the screw joint rotates.
Accordingly, the above mentioned study showed that the screw joint is tightened over a further angular distance after the torque peak has occurred, and that the actual screw tension in a vast majority of cases corresponds to a considerably lower torque level than the indicated peak level. Hence, the indicated peak torque level is not the same as the installed torque and does not truly reflect the tension in the screw joint. Accordingly, it is not useful as a process control measurement.
The primary object of the invention is to improve the accuracy of impulse tightening of screw joints by obtaining a more accurate measurement of the installed torque in the screw joint.
Another object of the invention is to accomplish an improved method for controlling a screw joint tightening process by using the new improved method for measuring the installed torque in the screw joint.
A still further object of the invention is accomplish an improved method for quality checking the end result of a screw joint tightening process by using the installed torque measurement in accordance with the new method as well as a measurement of the total angular movement of the joint.
Further objects and advantages of the invention will appear from the following detailed description of a preferred embodiment of the invention with reference to the accompanying drawings.